When an incident photon collides with a polyatomic gas molecule, it may either be scattered elastically, i.e., without energy exchange, or inelastically, i.e., with energy exchange that excites or de-excites a rotational/vibrational mode of the molecule. If the incident photon collides with a gas molecule and excites the gas molecule to a higher vibrational/rotational energy mode, the photon is re-emitted at a lower energy and consequently lower frequency than the incident photon. This inelastic scattering is termed Stokes Raman scattering. Similarly, if the incident photon collides with a gas molecule and de-excites the gas molecule to a lower vibrational/rotational energy mode, the photon is re-emitted at a greater energy and consequently higher frequency than the incident photon. This type of inelastic scattering is termed anti-Stokes Raman scattering. Although both effects may be observed, at room temperatures the Stokes Raman effect is generally more intense and thus, easier to measure. Therefore, the light resulting from collisions according to the Stokes Raman effect is typically analyzed and will be discussed herein.
Rotational/vibrational energy modes of molecules are quantized, forcing photons to exchange energy with molecules in discrete amounts. Different gas molecules require different amounts of energy from a photon to excite molecules to a higher rotational/vibrational mode. Thus, the amount of energy necessary to excite a gas molecule to a higher mode is characteristic of the type of molecule. The change in the photon's frequency caused by inelastic scattering corresponds to the amount of energy lost by the photon and can be used to identify the type of molecule which caused the scattering. Such analysis is called Raman spectroscopy.
Raman spectroscopy systems typically comprise a laser which directs intense, monochromatic light energy, toward a gas sample to be analyzed. Detectors, such as photo multiplier tubes or avalanche photodiodes, are arranged about the gas sample to receive Raman scattered energy. Filters remove elastically scattered energy at the wavelength of the laser source. Additional filters, each filter being designed to pass a different wavelength of expected Raman scattered energy, or a different Raman line, are placed in front of different detectors. As many detectors may be utilized as there are expected Raman lines. U.S. Pat. No. 4,784,486 to Van Wagenen et al., describes one type of multi-channel system. Alternatively, a rotating filter which passes different Raman lines as it rotates may be employed with a single detector. U.S. Pat. No. Re. 34,153 to Benner et al., describes one type of single channel system. The amount of energy collected on each detector corresponds to the concentration of the gas which created the particular Raman line.
Although there are filters between the gas sample and the detectors in the Raman spectroscopy system, light at wavelengths other than the lasing wavelength produced by the laser source, e.g., laser glow, is often collected by the detector, causing background noise in the measured signal. Additionally, photodetectors often have inherent dark noise which adds to the background noise. For accurate measurements of light due to Raman scattering, the system should be calibrated such that the signal from such background noise at the photodetectors is determined, i.e., a zero-calibration level of the system is set equal to the background noise level. Thus, there exists a need for apparatuses and methods for zero-calibration of Raman spectroscopy gas analysis systems.